In the metric system of measurements, the prefix "milli" means a thousandth of the base unit, which in this case is the gram. To convert from milligrams to grams, divide grams by 1,000, and vice versa. Conversion between grams and milligrams can become necessary when a dietary supplement is expressed in terms of the total recommended daily intake by health professionals, but at the drugstore it is available only in tablet form that is measured in milligrams. In that case, the individual has to determine how many tablets are needed to meet the daily required dosage of the supplement.